Measurement

ABSTRACT

Methods and apparatus are provided for measuring emissions from radioactive material in a matrix. Consideration is made in the variation in counts observed at different rational positions of the body of material so as to establish the information about the position of the radioactive material within the matrix.

This invention concerns improvements in and relating to measurement,particularly of emissions from radioactive material.

In a number of situations it is desirable to be able to quantify theamount of radioactive material within a package. The package may be acontainer into which waste material has been introduced, the waste beingassociated with unknown amounts of radioactive material. Othersituations also exist.

The existing instruments and methods include a level of error withinthem. These errors arise particularly from a lack of full accounting forthe effects of attenuation and/or the shape and/or the size of thesource.

The present invention has amongst its aims to reduce the level of errorin the measurement. It is amongst the aims of the present invention totake into account more fully the position of the radioactive materialwithin the package. It is amongst the aims of the present invention totake into account the suggested nature of the radioactive material, forinstance shape and/or size.

According to a first aspect of the invention we provide a method ofmeasuring emissions from radioactive material in a matrix, the methodcomprising

-   -   providing a detector for the emissions;    -   providing the matrix, at least in part, within the field of view        of the detector, the matrix having an axis of rotation;    -   measuring the emissions at a first orientation of the matrix        about the axis of rotation, the emissions and orientation        forming a first data set;    -   measuring the emissions at one or more further orientations of        the matrix about the axis of rotation, the emissions and        orientation at the one or more further orientations forming one        or more further data sets;    -   comparing the emissions detected in the first data set with the        emissions detected in one or more of the one or more further        data sets, information on the position of the radioactive        material relative to the axis of rotation being derived from the        variation in the emissions detected between data sets.

According to a second aspect of the invention we provide a method ofmeasuring emissions from radioactive material in a matrix, the methodcomprising

-   -   providing a detector for the emissions;    -   providing the matrix, at least in part, within the field of view        of the detector, the detector being rotatably about an axis, the        axis of rotation passing through the matrix;    -   measuring the emissions at a first rotational orientation of the        detector about the axis of rotation, the emissions and        orientation forming a first data set;    -   measuring the emissions at one or more further rotational        orientations of the detector about the axis of rotation, the        emissions and orientation at the one or more further        orientations forming one or more further data sets;    -   comparing the emissions detected in the first data set with the        emissions detected in one or more of the one or more further        data sets, information on the position of the radioactive        material relative to the axis of rotation being derived from the        variation in the emissions detected between data sets.

Preferably the axis of rotation is also perpendicular to the axis of thefield of view of the detector.

The first aspect and/or second aspect of the invention may include anyof the features, options or possibilities set out elsewhere in thisapplication, including those of the third and/or fourth and/or fifthand/or sixth aspects of the invention.

Preferably the measurement of emissions is used to measure the amount ofradioactive material in the matrix.

Preferably the matrix or detector is rotated through a series of furtherorientations and preferably a further data set is obtained for eachorientation.

Preferably the variation between the orientation giving the highestmeasurement of emissions and the orientation giving the lowest level ofemissions is considered and most preferably is used to provide theindication of the radial position of the radioactive material.

Preferably the measurement of the amount of radioactive material in thematrix is corrected by applying a correction to the measurement, thecorrection being given, for that radial position of the radioactivematerial, by a correction factor.

According to a third aspect of the invention we provide a method ofmeasuring the amount of radioactive material in a matrix by measuringemissions arising from the radioactive material, the method comprising

-   -   providing a detector for the emissions;    -   providing the matrix, at least in part, within the field of view        of the detector, the matrix having an axis of rotation;    -   measuring the emissions at a first orientation of the matrix        about the axis of rotation, the emissions and orientation        forming a first data set;    -   rotating the matrix about the axis of rotation through a series        of further orientations and measuring the emissions at each of        those further orientations to form further data sets;    -   the variation between the emissions detected at the orientation        which gives the highest measurement of emissions and the        orientation which gives the lowest measurement of emissions        indicating the radial position of the radioactive material        relative to the axis of rotation;    -   the measurement of the amount of radioactive material in the        matrix being corrected by applying a correction to the        measurement, the correction being given by a correction factor        for that radial position of the radioactive material.

According to a fourth aspect of the invention we provide a method ofmeasuring the amount of radioactive material in a matrix by measuringemissions arising from the radioactive material the method comprising

-   -   providing a detector for the emissions;    -   providing the matrix, at least in part, within the field of view        of the detector, the detector being rotatable about an axis of        rotation, the axis of rotation passing through the matrix;    -   measuring the emissions at a first orientation of the detector        about the axis of rotation, the emissions and orientation        forming a first data set;    -   rotating the detector about the axis of rotation through a        series of further orientations and measuring the emissions at        each of those further orientations to form further data sets;    -   the variation between the emissions detected at the orientation        which gives the highest measurement of emissions and the        orientation which gives the lowest measurement of emissions        indicating the radial position of the radioactive material        relative to the axis of rotation;    -   the measurement of the amount of radioactive material in the        matrix being corrected by applying a correction to the        measurement, the correction being given by a correction factor        for that radial position of the radioactive material.

Preferably the axis of rotation is also perpendicular to the axis of thefield of view of the detector.

The third and/or fourth aspect of the invention may include any of thefeatures, options or possibilities set out elsewhere in thisapplication, including those of the first and/or second and/or fifthand/or sixth aspects.

The amount of radioactive material maybe expressed as a mass.

The radioactive material may be or include uranium and particularlyU²³⁵. The radioactive material may be or include plutonium.

The matrix may be homogeneous. The matrix may be heterogenous. Thematrix may be known. The nature of the matrix may be determined as partof the method or a precursor thereto. The nature of the matrix,particularly its effect on the emissions, may be investigated using atransmission source. Preferably the effect of the matrix on emissionsfrom the transmission source is detected, potentially using the samedetector as for the emissions arising within the matrix. Preferably thelevel and/or characteristics of the transmission source are known.

The matrix may be contained, for instance provided within a package. Theeffect of the package may be considered as part of the matrix effect.The package may be a crate, drum or the like. The package may be sealedbefore, during and after the measurement is made.

The emissions may be gamma emissions. The emissions may be neutronemissions.

The detector may be a gamma detector. The detector may be a neutrondetector. One or more detectors of the same or different types may beprovided.

Preferably the matrix in at least two directions is entirely within thefield of view of the detector, ideally at all orientations. Only a partof the matrix need be within the field of view in a third direction,ideally at all orientations. The two directions may be perpendicular toanother and ideally to the third direction too. The two directions mayboth be perpendicular to the axis of rotation. The third direction maybe or be parallel to the axis of rotation. The method may be repeatedwith different parts of the matrix within the field of view,particularly by moving the matrix and/or detector relative to oneanother along the third direction and/or by providing one or morefurther detectors which have different fields of view spaced along thethird direction.

Preferably the axis of rotation is vertical. The axis of rotation ispreferably perpendicular to at least one surface of the matrix orpackage containing it, particularly the top and/or bottom surfacesthereof. The axis of rotation may be parallel to one or more surfaces ofthe matrix or package containing it, particularly the side wall orwalls.

Preferably a single axis of rotation is used in the method. Preferablythe same axis of rotation is used for packages of the same type.

Preferably the count rate is measured at the orientations.

The first orientation may be a first rotational position or first angle.The further orientations maybe further rotational positions or furtherangles. The matrix or detector may be rotated between differentorientations in a step like manner or more preferably by means ofcontinuous rotation. The first orientation may be a single position ormay be a range of positions. The first orientation may thus be an arcthrough which the matrix or detector rotates. Preferably the range ofpositions is of the same size and/or the arc is of the same size foreach orientation.

The emission measurement, particularly count rate, and orientation ofthe first and further data sets may be recorded.

Preferably the first orientation is next to a further orientation. Thefirst and further orientations may be next to one another in the form ofadjacent angles or may be next to one another in the form of sequentialarc ranges. Preferably each further orientation is provided with afurther orientation to each side of it, or a further orientation to oneside and the first orientation to the other. Preferably the first andfurther orientations are distributed throughout a range of at least 180°and more preferably 360°. Preferably the first and further orientationsprovide continuous coverage through a half or full circle about the axisof rotation.

Preferably the first and further orientations provide at least 20, morepreferably at least 50 and ideally at least 90 orientations in total.

The orientation giving the highest measurement of emissions and theorientation giving the lowest measurement of emissions may be 180°apart. Variation of zero, or under a small threshold, for instance athreshold equivalent to the noise encountered on the emissionmeasurement may be taken as indicative of a position for the radioactivematerial on the axis of rotation. Variation above a threshold may betaken as indicating a position at the outside of the matrix relative tothe axis of rotation. Variation intermediate to zero or the lowerthreshold and the upper threshold may be taken as an indication of aposition intermediate the axis of rotation and the outside of thematrix.

The position may be determined as a radial position, for instance adistance from the axis of rotation. Additionally, the position may bedetermined as an angular and radial position, for instance a distancefrom the axis of rotation and an angle relative to a reference angle.The reference angle may be linked to a feature on the matrix or packagecontaining it.

A plurality of different positions may be indicated for different piecesof radioactive material.

The measurements from one or more of the orientations, preferably all,may be used to provide an uncorrected measurement of the amount ofradioactive material in the matrix, particularly that part of the matrixwithin the field of view of the detector. The uncorrected measurementmay be a measured mass of material.

Preferably the uncorrected measurement may be corrected upward ordownward or remain the same according to the position and/or size of thematerial measured.

The correction may be the correction given by the correction factor forthe radial position determined. The correction may be the correctiongiven by the correction factor for the radial and angular positiondetermined. The correction may be the correction given for the radialposition and/or material size determined.

The form of the correction factor may be determined by a modellingprocess and/or by a calibration process for the method and particularlyfor the instrument and/or instrument type used to perform the method.The form of the correction factor may be determined by determining themeasured amount and/or measured mass obtained for one or morecombinations and the extent of correction needed to correct the measuredamount or mass to be the same as the actual amount or mass in a givencombination. A combination may comprise a radioactive material sizeand/or radioactive material radial position and/or radioactive materiallongitudinal position for one or more pieces of radioactive material.The radioactive material size may be a point source and/or may be asource exhibiting self-shielding.

The determination of the form of the correction factor may include theconsideration of at least 500 combinations, more preferably at least2000 combinations and ideally at least 10000 combinations. Preferablythe combinations are randomly selected in terms of radioactive materialsize and/or radioactive material physical size and/or radioactivematerial activity and/or radioactive material radial position and/orradioactive material longitudinal position and/or number of pieces ofradioactive material.

Preferably a form for the correction factor is derived from theconsideration of the combinations and is preferably adjusting in formuntil a statistically acceptable fit is achieved.

Different forms of the correction factor may derived for different typesof package and/or matrix and/or radioactive material and/or size ofradioactive material. The form of the correction factor may assumeradioactive material is present as a point source. The form of thecorrection factor may allow for some or all of the radioactive materialto be present in self-shielding forms, potentially with differentextents of self-shielding.

The correction factor for self-shielding situations may treat theradioactive material as being present in sizes according to adistribution. The distribution may be an exponential distribution,preferably biassed towards the occurrence of point source forms of theradioactive material. The distribution may cause the correction factorto apply a weighted correction to the measured amount or mass.

Preferably the application of the correction converts the measuredamount or mass into a corrected amount or mass.

According to a fifth aspect of the invention we provide apparatus formeasuring emissions from radioactive material in a matrix, the apparatuscomprising

-   -   a detector which produces signals in response to the detection        of the emissions;    -   signal processing electronics for the signals, the signal        processing electronics including a time allocator which        allocates a time of detection indication to sets of one or more        signals;    -   a measurement location, on which the matrix is provided in use,        the measurement location being capable of rotation about an axis        of rotation so as to present the matrix to the detector at a        first orientation of the matrix about the axis of rotation and        at one or more further orientations of the matrix about the axis        of rotation, a set of signals being those signals arising at a        particular orientation;    -   a comparator for comparing the signals of one set with the        signals of one or more of the other sets, information on the        position of the radioactive material relative to the axis of        rotation being derived from the variation in the signals arising        between sets.

According to a sixth aspect of the invention we provide apparatus formeasuring emissions from radioactive material in a matrix, the apparatuscomprising

-   -   a detector which produces signals in response to the detection        of the emissions;    -   signal processing electronics for the signals, the signal        processing electronics including a time allocator which        allocates a time of detection indication to sets of one or more        signals;    -   a measurement location, on which the matrix is provided in use,        the detector being capable of rotation about an axis of rotation        and about the matrix so as to present the matrix to the detector        at a first orientation about the axis of rotation and at one or        more further orientations about the axis of rotation, a set of        signals being those signals arising at a particular orientation;    -   a comparator for comparing the signals of one set with the        signals of one or more of the other sets, information on the        position of the radioactive material relative to the axis of        rotation being derived from the variation in the signals arising        between sets.

Preferably the axis of rotation is also perpendicular to the field ofview of the detector.

The apparatus preferably also includes a processor to calculate and/orallocate a correction to the measurement of the amount of radioactivematerial in the matrix, the correction being given by a correctionfactor for that radial position of the radioactive material.

Preferably the measurement location is capable of rotational and axialmovement. The rotational movement may occur together with axialmovement.

The information on the position on the radioactive material relative tothe axis of rotation may be expressed as a position on a plane extendingabout the axis of rotation and/or perpendicular to an axis extendingbetween the measurement location and detector.

The fifth and/or sixth aspect of the invention may include any of thefeatures, options or possibilities set out elsewhere in thisapplication, including those of the first and/or second and/or thirdand/or fourth aspects.

Various embodiments of the invention will now be described, by way ofexample only, and with reference to the accompanying drawings in which:

FIG. 1 is a schematic illustration of the impact of radioactive materialposition and size on measurement;

FIG. 2 is a schematic representation of position determining techniqueof the present invention;

FIG. 3 illustrates the variance in count rate with rotation of a packagefor different sources;

FIG. 4 is a In plot of the proposed correction factor against fullcorrection factor for a point source in a package;

FIG. 5 is a In plot of uncorrected mass against corrected mass, usingthe proposed correction factor, for a point source in a package;

FIG. 6 is a In plot of proposed correction factor against fullcorrection factor for a self absorbing source in a package; and

FIG. 7 is a In plot of uncorrected mass against corrected mass, usingthe proposed correction factor, for a self absorbing source in apackage.

The determination of an estimate of the mass of radioactive material ina package is a desirable aim for a variety of reasons. In a commonsituation there is a need to determine the mass of radioactive materialin a package so as to establish the appropriate subsequent handling andstorage of the package. Such measurements are also used to control thefilling of larger containers with a view to criticality control.

A problem with such measurements is that the radioactive material formsonly a small part of the content of the package. The remaining material,the matrix, has an impact on the measurements made. Additionally, thesize and/or shape of the radioactive material itself also has an impact.

With regard to the matrix effect, the amount of matrix between thematerial and the detector of the instrument influences the count ratefrom a given mass of material. Thus, in FIG. 1, the greater amount ofmatrix 20 between the detector 21 and the source 22 gives a lower countrate from a given mass of material compared with a source 22′, as thelevel of attenuation of the emissions is higher.

To try and address this issue in existing instruments, rotation of thepackage occurs during the measurement process. The intention is that bytaking the total count for the measurements made throughout therotation, the risk of a large scale mis-measurement of material near theedge of the package is reduced. Otherwise, if a measurement were made ata single position it would be almost inevitable that the position of thematerial would skew the measurement. Thus if the material were at aposition on the far side of the package a low result would occur,whereas if the material were at the nearside a high result would occur,in these and other cases a non-representative measurement would result.The measurement with rotation is made for each of a series of verticalpositions relative to the detector. As a consequence, a series of slicesthrough the package are effectively considered, with the total count forthat slice being used in the calculation of the estimated mass for thatslice.

With regard to the size of the radioactive material, the amount ofmaterial in a discrete mass 24′ or in very close proximity 24″ issignificant, when compared with a point source 22. Materials in asignificant mass, 24′ or 24″, have a self-shielding capacity and thisbecomes sizable at even small discrete masses. Thus a lump of 1 g ofmaterial would give a significantly lower count rate and hence measuredmass, than 1 g of material distributed throughout a significant part ofthe matrix.

The prior art approach does not fully account for the problems caused bythe position of the material, makes no estimation of the materialsradial position and does not account for material size and/or shape.Hence a number of error sources for the measured mass arise. The levelof the error varies with the emissions being considered, particularlytheir energy. The problem is particularly significant with respect tolow energy emissions, such as the 186 keV energy of U²³⁵ gammaemissions.

In the improved technique of the present invention, the measurement ofthe count rate from the package is synchronised with the rotation of thepackage. In an alternative form, particularly suitable for largepackages, the detector may be rotated about the package, for instance ona gantry. As a consequence, the count rate will vary with rotation in amanner which is different for different positions within the package.This allows the radial position of the material and/or its actualposition within a package to be determined.

The principle is illustrated in the schematic plan view of FIG. 2. Inthis case, the package is a drum 30 which is considered whilst on aturntable 32 which can be rotated. The detector 34 for the highresolution gamma spectroscopy is provided to one side of the drum 30. Inthe first case the drum 30 contains the material in the form of a firstsource 36 which is close to the edge of the drum 30 and of relativelylow activity. As the measurement process starts the source 36 is on thefar side of the drum 30 and of relatively low authority. As aconsequence, the attenuation effect of the matrix 38 within the drum 30is at its greatest as the distance X through the matrix 38 is at itsgreatest. This is reflected in the count rate plot A of FIG. 3, wherethe count rate starts at a low level, angle 0°.

As the drum 30 rotates and the angle increases, the source 36 atposition 36′ comes closer to the detector 34 and the distance throughthe matrix 38 decreases, distance X′. The result is an increase in thecount rate for that position, see plot A of FIG. 3. Further rotationcauses the distance to decrease and the count rate to go up. Once thesource 36 passes the closest point, angle 180°, and the distanceincreases again the count rate declines.

The variation in count rate with rotation is high when the separation ofthe source 36 from the axis of rotation is great. In the case of adifferent source case, 39, that source 39 is at a position much closerto the axis and hence the variation in the matrix thickness between thesource 39 and the detector with rotation is much less. The variation inthe count rate with rotation is less as a result and such a case isillustrated in the alternative count rate, plot B, in FIG. 3. The highercount rate is a reflection of source 39 being larger than source 36. Asource on axis of rotation may well give no variation with rotation.

Different radial positions for the material detected are reflected interms of the different shapes of the count rate plots with rotationalangle. The extent of the radial distance between the source positiondetermined and the axis can be used as the basis for the correctionapplied. Different masses are reflected in terms of the different countrate levels. Differences in the radial and angular position of thesource are reflected in the different shapes of the count rate plots andthe different angle at which the maximum and minimum count rate occurswithin that plot respectively. The extent of the radial distance betweenthe source position determined and the axis, together with the angularposition could be used as the basis for the correction applied.

Once the position of the material of the source has been determined anappropriate correction can be applied to accurately correct for thatposition of the material as opposed to other positions. The inventionprovides for such a positional determination for the first time and forsuch positional correction for the first time. The correction appliedreflects the variation in measurement efficiency of the system for thatposition. The correction factor is based on the variation in measurementefficiency with position. This efficiency with position can bedetermined as part of the calibration process for the instrument. Thepositional information reflects the longitudinal position relative tothe axis of rotation in terms of the slice to which the source isallocated and the radial position of the source. The absolute positionin terms of longitudinal position, radial position and angular positioncan form the positional information.

The correction for position involves the application of a correction tothe measured mass to give the corrected mass; the aim being that thecorrected mass should be the same as the actual mass present or at leastas close as possible thereto. The correction for a position is set bythe value of a correction factor for that position.

The correction factor takes the general formln(SDCF)=F(V _(L) ,V _(A) ,W _(L) ,W _(A))

where SDCF is the source distribution correction factor—the correctionapplied for a given position; F is an empirical correction factor; V_(L)is the longitudinal detector response variance; V_(A) is the angulardetector response variance; W_(L) is the longitudinal detectortransmission response efficiency, and W_(A) is the angular detectortransmission response efficiency.

To determine the form of the correction factor, a substantial number ofknown sources and known positions were investigated. This process wasperformed for both point sources and larger sources and involved theconsideration of around 15000 combinations in each case. For aparticular combination the count rates lead to a measured mass. This canbe compared with the actual mass and hence a correction to convert themeasured mass to a corrected mass matching the actual mass determined.The outcome of this consideration for all of the combinations leads to ageneral form of the correction factor.

FIG. 4 is a plot of the correction applied to take the measured mass toa corrected mass, for a large number of combinations, with thecorrection defined by the correction factor, against the actualcorrection needed to give a fully corrected mass which matches theactual mass. The plot indicates that for a wide range of situations thecorrection factor provides a correction very close to the actualcorrection needed.

In general, the correction needs to reduce the value for those sourcesnear the outside of the drum (the left hand side of the plot) and toincrease the value for those sources near the centre of the drum (theright hand side of the plot). The combinations for which the greatestlevel of discrepancy between the correction suggested by the correctionfactor and actual correction required exist are those for which thematrix is of high density. Measurement for such matrices is even moreproblematic using prior art approaches. At matrix densities below 1g.cm⁻³ the correlation between correction proposed according to thecorrection factor and the actual correction is very good indeed. Thisperformance is borne out in FIG. 5 where the measured mass for a seriesof combinations is plotted against the actual mass (red colour/dotsplots) and is compared with the corrected mass plotted against theactual mass (black colour/crosses plots).

Whilst the performance of the present invention in respect of pointsources is demonstrated above, the technique can also be used to handlesources which are not point and which as a result have a self-shieldingeffect.

A similar principle is involved in reaching the form of the correctionfactor in this embodiment of the invention to that used for pointsources. Again a very large number of combinations was considered,15000, with single point, multiple point, single “lump” and multiple“lump” sources present in a full range of different positions and sizesof source. Again the measured and actual masses were considered and acorrection determined. The results from all the combinations were usedto produce a general form for the correction factor.

In FIG. 6, for cases with a matrix of 1 g.cm⁻³ or less, the correctionsuggested by the correction factor is plotted against the actualcorrection needed to give a fully corrected mass which matches theactual mass.

In this case, point sources require relatively low correction, where aslarge “lumps” require substantial correction to account for theself-shielding effect.

When considering the correction to apply, the nature of the materialpresent needs to be determined or assumed.

In one approach, a particle size distribution is assumed to apply to thematerial in the matrix. As a consequence the correction determinedaccording to the correction factor is weighted according to thatdistribution. Thus if the distribution indicates a high likelihood oflump sources a predominantly lump source appropriate correction isgenerated by the correction factor and is applied. In practice, apredominance of smaller sources may be likely and hence an exponentialdistribution biassed in that way would be used. Other distributions maybe applied according to other likely situations, knowledge of sourcesizes etc.

In FIG. 7, the exponential distribution approach is used and a plot ofmeasured mass against actual mass (red colour/dot plots) and correctedmass against actual mass (black colour/crosses plots) is provided. Thecorrected mass has a standard error of ˜30% compared with up to twoorders of magnitude uncorrected.

The technique set out above is applicable to the correction of a widevariety of emission types and in particular to gamma emissions andneutron emissions and instruments which measure them.

The correction applied in its preferred form will also include anaccount of the matrix type, as different matrices exhibit asignificantly different attenuation effect to one another. To establishthe matrix in question, a transmission source, whose characteristics interms of emissions are known, is used. The emissions from thetransmission source pass through the package and hence matrix prior todetection. The impact of the matrix on the transmitted emissions can beestablished by those emissions actually detected and this leads to aproposed matrix type and as a consequence correction.

1-21. (canceled)
 22. A method of measuring emissions from radioactivematerial in a matrix, the method comprising: providing a detector forthe emissions; providing the matrix, at least in part, within the fieldof view of the detector, the matrix having an axis of rotation;measuring the emissions at a first orientation of the matrix about theaxis of rotation, the emissions and orientation forming a first dataset; measuring the emissions at one or more further orientations of thematrix about the axis of rotation, the emissions and orientation at theone or more further orientations forming one or more further data sets;and comparing the emissions detected in the first data set with theemissions detected in one or more of the one or more further data sets,information on the position of the radioactive material relative to theaxis of rotation being derived from the variation in the emissionsdetected between data sets.
 23. A method according to claim 22 in whichthe matrix is rotated through a series of further orientations and afurther data set is obtained for each orientation.
 24. A methodaccording to claim 22 in which the variation between the orientationgiving the highest measurement of emissions and the orientation givingthe lowest level of emissions is considered and is used to provide theindication of the radial position of the radioactive material.
 25. Amethod according to claim 22 in which the measurement of the amount ofradioactive material in the matrix is corrected by applying a correctionto the measurement, the correction being given, for that radial positionof the radioactive material, by a correction factor.
 26. A method ofmeasuring emissions from radioactive material in a matrix, the methodcomprising providing a detector for the emissions; providing the matrix,at least in part, within the field of view of the detector, the detectorbeing rotatably about an axis, the axis of rotation passing through thematrix; measuring the emissions at a first rotational orientation of thedetector about the axis of rotation, the emissions and orientationforming a first data set; measuring the emissions at one or more furtherrotational orientations of the detector about the axis of rotation, theemissions and orientation at the one or more further orientationsforming one or more further data sets; and comparing the emissionsdetected in the first data set with the emissions detected in one ormore of the one or more further data sets, information on the positionof the radioactive material relative to the axis of rotation beingderived :from the variation in the emissions detected between data sets.27. A method according to claim 26 in which the detector is rotatedthrough a series of further orientations and a further data set isobtained for each orientation.
 28. A method according to claim 26 inwhich the variation between the orientation giving the highestmeasurement of emissions and the orientation giving the lowest level ofemissions is considered and is used to provide the indication of theradial position of the radioactive material.
 29. A method according toclaim 26 in which the measurement of the amount of radioactive materialin the matrix is corrected by applying a correction to the measurement,the correction being given, for that radial position of the radioactivematerial, by a correction factor.
 30. A method of measuring the amountof radioactive material in a matrix by measuring emissions arising fromthe radioactive material, the method comprising providing a detector forthe emissions; providing the matrix, at least in part, within the fieldof view of the detector, the matrix having an axis of rotation;measuring the emissions at a first orientation of the matrix about theaxis of rotation, the emissions and orientation forming a first dataset; rotating the matrix about the axis of rotation through a series offurther orientations and measuring the emissions at each of thosefurther orientations to form further data sets; the variation betweenthe emissions detected at the orientation which gives the highestmeasurement of emissions and the orientation which gives the lowestmeasurement of emissions indicating the radial position of theradioactive material relative to the axis of rotation; the measurementof the amount of radioactive material in the matrix being corrected byapplying a correction to the measurement, the correction being given bya correction factor for that radial position of the radioactivematerial.
 31. A method according to claim 30 in which the nature of thematrix, particularly its effect on the emissions, is investigated usinga transmission source.
 32. A method according to claim 30 in which theaxis of rotation is vertical.
 33. A method according to claim 30 which asingle axis of rotation is used in the method.
 34. A method according toclaim 30 in which the first and further orientations are distributedthroughout a range of at least 180°.
 35. A method according to claim 30in which the position is determined as a radial position.
 36. A methodaccording to claim 30 in which the position is determined as an angularand radial position.
 37. A method according to claim 30 in which anglesare expressed relative to a reference angle and the reference angle islinked to a feature on the matrix or package containing it.
 38. A methodaccording to claim 30 in which the measurements from one or more of theorientations are used to provide an uncorrected measurement of theamount of radioactive material in the matrix.
 39. A method according toclaim 38 in which the uncorrected measurement is corrected upward ordownward or remains the same according to the position and/or size ofthe radioactive material measured.
 40. A method according to claim 39 inwhich the correction is the correction given by the correction factorfor the radial position determined or the correction is the correctiongiven by the correction factor for the radial and angular positiondetermined or the correction is the correction given for the radialposition and/or material size determined.
 41. A method according toclaim 40 in which the form of the correction factor is determined by amodeling process and/or by a calibration process for the method.
 42. Amethod of measuring the amount of radioactive material in a matrix bymeasuring emissions arising from the radioactive material the methodcomprising providing a detector for the emissions; providing the matrix,at least in part, within the field of view of the detector, the detectorbeing rotatable about an axis of rotation, the axis of rotation passingthrough the matrix; measuring the emissions at a first orientation ofthe detector about the axis of rotation, the emissions and orientationforming a first data set; rotating the detector about the axis ofrotation through a series of further orientations and measuring theemissions at each of those further orientations to form further datasets; the variation between the emissions detected at the orientationwhich gives the highest measurement of emissions and the orientationwhich gives the lowest measurement of emissions indicating the radialposition of the radioactive material relative to the axis of rotation;and the measurement of the amount of radioactive material in the matrixbeing corrected by applying a correction to the measurement, thecorrection being given by a correction factor for that radial positionof the radioactive material.
 43. A method according to claim 42 in whichthe nature of the matrix, particularly its effect on the emissions, isinvestigated using a transmission source.
 44. A method according toclaim 42 in which the axis of rotation is vertical.
 45. A methodaccording to claim 42 which a single axis of rotation is used in themethod.
 46. A method according to claim 42 in which the first andfurther orientations are distributed throughout a range of at least180°.
 47. A method according to claim 42 in which the position isdetermined as a radial position.
 48. A method according to claim 42 inwhich the position is determined as an angular and radial position. 49.A method according to claim 42 in which angles are expressed relative toa reference angle and the reference angle is linked to a feature on thematrix or package containing it.
 50. A method according to claim 42 inwhich the measurements from one or more of the orientations are used toprovide an uncorrected measurement of the amount of radioactive materialin the matrix.
 51. Apparatus for measuring emissions from radioactivematerial in a matrix, the apparatus comprising a detector which producessignals in response to the detection of the emissions; signal processingelectronics for the signals, the signal processing electronics includinga time allocator which allocates a time of detection indication to setsof one or more signals; a measurement location, on which the matrix isprovided in use, the measurement location being capable of rotationabout an axis of rotation so as to present the matrix to the detector ata first orientation of the matrix about the axis of rotation and at oneor more further orientations of the matrix about the axis of rotation, aset of signals being those signals arising at a particular orientation;and a comparator for comparing the signals of one set with the signalsof one or more of the other sets, information on the position of theradioactive material relative to the axis of rotation being derived fromthe variation in the signals arising between sets.
 52. Apparatus formeasuring emissions from radioactive material in a matrix, the apparatuscomprising a detector which produces signals in response to thedetection of the emissions; signal processing electronics for thesignals, the signal processing electronics including a time allocatorwhich allocates a time of detection indication to sets of one or moresignals; a measurement location, on which the matrix is provided in use,the detector being capable of rotation about an axis of rotation andabout the matrix so as to present the matrix to the detector at a firstorientation about the axis of rotation and at one or more furtherorientations about the axis of rotation, a set of signals being thosesignals arising at a particular orientation; and a comparator forcomparing the signals of one set with the signals of one or more of theother sets, information on the position of the radioactive materialrelative to the axis of rotation being derived from the variation in thesignals arising between sets.